In recent years, in accordance with, for example, the widespread use of electric cars or the introduction of smart meters and power visualization systems, the market for current sensors for these purposes is rapidly increasing. The major four types of current sensor used by the market include (1) a shunt resistance method, (2) a current transformer method, (3) a Hall effect magnetic current sensor, and (4) a magnetic current sensor based on a magnetoresistance (MR) method.
Each of these sensors has limitations. In particular, the current sensor of (1) has a problem in that electric insulation is not easy. Moreover, with regard to the current sensor of (2), theoretically, only an alternate current can be measured. With regard to the current sensor of (3), reproducibility is poor since there exists magnetic hysteresis and the size of the current sensor needs to be increased since the sensitivity of a Hall element is lower than that of an MR device. Finally, with regard to the current sensor of (4), reproducibility is poor since a barber-pole MR device is used and, hence, there exists magnetic hysteresis.
FIG. 1 illustrates how magnetic hysteresis causes hysteresis to be generated in the output voltage of a magnetic current sensor. In the magnetic current sensor, a difference in output voltage is generated between the upward current direction and downward current direction due to, for example, magnetic hysteresis of an MR device being used. This difference constitutes the hysteresis of the output voltage of the magnetic current sensor. In order to suppress the magnetic hysteresis causing this phenomenon, Patent Document 1 discloses a method of applying a bias magnetic field to the MR device.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-148301.
Further enhancement is required in current sensors used in new areas such as electric cars, smart meters, power visualization systems, and the like. In other words, first, further enhancement in the linearity of an output voltage for an input current is required. Second, a function for accurate determination of the polarity (flow direction) of an input current is required. Third, a decrease in the hysteresis of an output voltage between the case in which the input current changes from a low current to a high current (called an up direction) and the case in which the input current changes from a high current to a low current (called a down direction) is required. Fourth, a further increase in current and a further reduction in size are required.